Hmm... my theory is that they'll reintroduce the Wii U to everyone, this time with more final hardware, and generally recap with new promo videos about what it can do(Asymmetrical gameplay, wireless tablet streaming, touch, tv-less play, nfc, technical demos, etc.), as well as unveil any additional small improvements/features that would otherwise take up too much time at the press conference, like analog sticks, multi-tablet support, etc.
So basically a retread of the E3 2011 reveal, but more complete since it reveals some mini-bombs.